Fluent material level control system

ABSTRACT

A system for detecting, indicating and controlling the level of a fluent material in a container. This system applies alternating current to two electrodes in a container, the electrically conducting paths through the electrodes to the container being included in a voltage divider network. A solid-state circuit controls a pump and motor in response to the output of the voltage divider network to regulate the level of the material in the container.

United States Patent 3 1 3,602,251

[72] Inventor John Cnrrolllllll [56] References Cited UNITED STATES PATENTS [m P 3,279,379 10/1966 Klyce ..340/244cux [221 Filed .Iuly22, 1969 3,496,380 2/1970 Jones... 137/392x hum! 3509 s25- 5/1970 Sorensen 137/392x [731 Assignee Standard International Corporation Andover, Mm. Primary Examiner-Alan Cohan Assistant ExaminerDavid R. Matthews Attorney-Joseph Weingarten ABSTRACT: A system for detecting, indicating and con- [54] FLUENTMATERIAL LEVEL CONTROL SYSTEM trolling the level of a fluent material in a container. This 10 2 Dnwlng Figs system applies alternating current to two electrodes in a con- [52] 1.8. CI 137/392, tainer, the electrically conducting paths through the elec- 3 l7/DIG. 3, 324/65 P trodes to the container being included in a voltage divider net- [51] Int. F 16k 21/18 work. A solid-state circuit controls a pump and motor in [50] Field of Search 137/386, response to the output of the voltage divider network to regu- 387, 392; 340/244, 244 C late the level of the material in the container.

i g i l RESERVOR I L I 7 l l i #32 l J g l l 4'" 54 l l 57 l l I l 5e l L a l '1' ill- PATENTEU AUG31 I971 sum 2 OF 2 I I I l I J I I L INVENTOR. J HN CARROLL HILL M ATTORNEYS between two predetermined values.

' some other level-adjusting dev ce.

- tain the level of the predetermined values.

. nor-2m com-nor. im

FIELD or THsmvsNT oN This invention relates in general to a level detectiomindica tion and control system and more particularly concerns a solid-state electronic system for regulating the level oi" a material within a container to maintain the material level n scussloisor THEPRIOR ART of material level is commonly accomplished by any one o'r'a combinationfof-fseveral means; Suchmeasurement means may include ultrasonic, radiation, potentiometric,

-capacitance,.or float-operated systems. .fl'he output ofthese such consumer market applications employs; an *electromechanical relay and associated moving cont'acts'in conjunction with two'elect'rodes imm'ersed in the liquid. A- significant drawback of this device is the use of the movingparts' associated with the electromechanical relay. Some similar devices using immersed el'ectrodeshave suffered fromthe fact that direct current is used on-thb' electrodes resulting in their corrosion due to electrolysis; causingthe eventual destruction otthesystem; r t

SUMMARY or- -"msn vsn noni Broadly speaking. the present-invention eomprises'a solidstate'electronic le'jvelcont'r'ol systemhaving anoperating life of I several years with high reliability'through. several} million operating cycles. The .systemlinclud'es a voltage div'iderfri'eti work including two electrodes suspended at different levels within a containerrhaving -.an electrically conductive wall in contact with the material-thereimIlre material whose level is to be controlled may be liquid or solid, the only requirement; being-that it be electrically conductive and readily flowable.

The material willbe generally referred to herein as a liquid. A n alternatingcurrentis applied to the voltage divider network which includes the electrodes and the container having liquid therein. Thealternating current output of the voltage .One system which has enjoyed some measure, of success for smalli volt'age exists in'the'liquid between the electrodes and the-container wall and the outputof thevoltage divider network is also small. However, these small voltages whose magnitudes are dependent upon liquid level are used to control full line voltageto the load. Furthermore, the invention is substantially immune to most line voltage fluctuations. This systemis so constructed that small differences in the output voltageof the voltage divider network-caused by intermediate changes in the'levelof the liquid do'not affect the'operationof the system. Furthermore. thesensitivity' of-thesystem need not be' exceptional because itis intended to react only to widely separated voltage levels. The use of such unambiguous signals results ina highly stable and reliable system.

Although thissystem' isuseful for'many purposes, it is specifically applicable to beverage-vending machines using carbonated :wate r. Further, the uniquecircuit disclosed may be used for indicating high' and low-liquid levels it; desired, rather than controlling the liquid to maintain it between two specified levels. Y

Bares nascalp'rlonor THE DRAWING A complete; understanding of theinve nti on and itslfeatures and advantages will become apparent from the following detailed description "taken. in conjunction with'the accompanyingdrawing wherein: v I FIG. 1 is a block diagram of ajsy tern-constructed in accorda'nce with the principles of this inventiom'and I FIG. '2 is a'schematic'di'agram of the system shown in FIG. 1.

' osscrunnon oF -E-premi /1 0mm -With reference now to thedrawingand more particularly to FIG. I thereof, there is. shown a blockdiagram of alevel control system powered by a conventional source of alternating current such as commonl l 5 v., 60 Hz. electrical power, as indicated by power source 1']. Voltage-divider 12 includes the effective-impedances of liquid 23 between the walls of condivider network is rectified and't'iltered and applied to the govern the operation of a power control device which is coninput of a threshold ,detector'such as a Schmitt 'trigger.-Th'is;

detector circuitresponds to certain levels of input voltage to nected across the input powerline and which in turn controls the power applied to a-load. Such a :power control device may be a triode AC-switch; cornmonlyrefe'rredto as a triac,-or a bidirectional silicon controlled'rectitier. The load, which may be a motor and pump,"or'-'simply a valve-controlling device,-

controls thesupply of liquidto jth'e'con tainer to therebymain- The level control system constructed ciples of this invention ha's'several advantages-over the-prior cordingtothe-prinliquid in'the container between two art. It is all"solid*sta "so'therefiare-nomoving-parts to wear out and norelay contacts with theirconco'mitant'physicaI-contac tand arcing problems. Witlralternatingcurrent-applied to f the electrodes;- no electrolysikcaused corrosion of the.elec-' trodes occurs. even during extended o'per'a'tingperiods. Qn lya taine'r-lsand electrodes l3, l4. Container 15 is convention'ally connected to a fluid reservoir 16 by supply pipe 17 and-has-an outlet 21 forsupplying fluid to other'apparatus, not

shown. Fluid 23 within container 15 is electrically conductive,

as are the inner surfaces of the walls of the container, which connected to the common, or groundfline 24 by means-of conductor 25. his thus evident that the input to voltage divider i2 is an alternating voltage as is the potential between electrodes '13, 14 andcontainer 15. The output of voltage divider 12, whichis also an alternating voltage, is applied to a-filtered'rectifieror demodulator circuit 26.'The output of the demodulator is' i-a direct" current voltage proportional to the peak output voltage of the voltage divider at any given time and is applied to a threshold detector'circuit 27 which has hysteresis-type characteristics. The'operating power for the detector circuit 27 is provided by power supply 31* whichis also connected across the AC power source. The output of threshold'detector 27 is then applied to a power control means 32 which is in series with load 33 across power source 1 l. It can'thus be seen from FIG. I that a relatively small-voltage'a'cross electrodes 13, Min container 15 is used to vary'the output of voltage divider l2 whichindirectly controls the operation of load 33. Load 33 may be a motor and pump which in turn control the supply of liquid from'reservoir 1610 container 15.

The'system of FIG. -1 is shown morespecifically'in the schematic of FIG; 2. Voltage divider 12 comprises resistor 34 in series' with the remainder of the voltage divider network which comprises three jparallel branches. One branch includes resistor 35; another branch includesan electrically'conductive path through highlevel electrode 14 and liquid.23,to eontainer 15, the third branch including a conductive path through resistor 36, low-level electrode '13 andliquid 23 to containerl5.'With the output of voltage divider networlt 12 being taken-across the above specified parallel combination, it

is apparent that this output varies proportionallyto the changes in output voltage of the network which occur as the level of liquid 23 rises or drops to cover or uncover the ends of electrodes l3, 14. This output voltage then becomes the input of demodulator 26.

Dernodulator 26 is shown as a standard linear'envelope detector comprising diode 37 capacitor 38 and resistor 39. The DC output of demodulator 26 is substantially proportional to the AC output of voltage divider 12 over the operating range of the system. The output voltage level of the voltage divider is low and the output of demodulator 26 is commensurately low. It will be recognized that since the output of the demodulator is applied to the base of a transistor in the threshold detector, its magnitude and the magnitude of the voltage changes need not be great, although the relative differences between the voltages may be significant.

The demodulator output voltage is applied to the input of the threshold detector 27 which is a modified, inverted, collector-base coupled binary, a type of Schmitt trigger having transistors 41, 42. The base of transistor 42 is coupled to the collector of transistor 41 through resistor 43 while the base of transistor 41 is coupled to the collector of transistor 42 through resistor 44. The threshold detector also includes resistors 45, 46, 47 and 48. As previously indicated, threshold detector 27 exhibits hysteresis, with the particularly hysteresis loop necessary for this operating sequence obtained by overcoupling the circuit. The degree of positive feedback, which is governed primarily by the choice of resistance values for resistors 44 and 45, determines the width of the hysteresis loop. Changes in the value of resistance of resistor 46 shifts the hysteresis loop between more positive or more negative input voltages without appreciably affecting the width of the hysteresis loop. Resistor 43 is primarily a current-limiting device to protect transistor 42 while resistor 47 is the collector load for transistor 41. Resistor 48 which is in the input circuit to power control means 32, is the collector load of transistor 42'. Power supply 31 comprising transformer 51, diode 52 and capacitor 53 provides the common supply voltage level for the emitters of transistors 41 and 42. Power supply 31 is shown as a conventional DC power supply, but various other means may be used to provide the desired supply voltage to the transistors.

in order to further explain the property of hysteresis exhibited by threshold detector 27, it must be understood that the input voltage which triggers the circuit either on or off depends not only upon the value of that voltage, but also upon whether that voltage is increasing or decreasing. That is, when the threshold detector is off, the input voltage must rise to a certain predetermined level before the circuit is triggered into operation. If, after the circuit is turned on, the input voltage decreases to a value less than the predetermined triggering value, the threshold circuit remains in an ON condition until a second, lower predetermined voltage level is reached. The threshold circuit is then turned off and will not be turned on again until the first predetermined higher input voltage is again reached. This is standard operation for a Schmitt trigger and is more fully described in various textbooks such as Pulse and Digital Circuits by Millman & Taub (1956).

When threshold detector circuit 27 is in an ON condition, a signal, which may be termed a gating or indicating signal, is fed to power control means 32 through resistor 48. Power control means 32 may consist of a triac 54 shunted by the series combination of resistor 55 and capacitor 56. A triac is a semiconductor device often used for alternating current switching. This device is capable of switching large currents in response to relatively small input triggering voltages applied to gate 57. Because the load of motor 33 in series with power control means 32 is generally inductive, resistor 55 and capacitor 56 are necessary to protect triac 54 from voltage surges. it should be recognized that the power control means may alternatively comprise a pair of inverse parallel-connected silicon controlled rectifiers.

' As shown in the drawing, motor 33 is used to control the supply of fluid from reservoir 16 to container in some conventional way such as by means of a pump (not shown) located within, or connected to, reservoir 36. it should be noted that this system provides means for maintaining the liquid within the container between certain predetermined levels by means of a motor 33 and that the operational state of the motor does not depend uniquely upon material level, that is, when only the low level electrode is immersed, the motor may or may not run depending upon the most recent history of the liquid level. The hysteresis loop of the threshold detector prevents the motor from running each time fluid is drained from container 15;,and provides for operation of the motor only when the fluid level reaches a certain predetermined low point and turns off the motor when the level has reached a predetermined high point. The length of time motor 33 operates during each cycle depends, of course, upon the size of the container and upon the capacity of the pump supplying fluid to the container.

The particular configuration of the level control system shown in the drawing has a very important advantage in that it is substantially immune to normal line voltage fluctuations with respect to its control operation. For example, if the line voltage increases, the demodulator output increases, providing an increase'to the input of the threshold circuit. However, the threshold voltage applied to the emitters of transistors 41 and 42 by power supply 31 increases proportionately so that the trigger points of the hysteresis loop are substantially unchanged by such line voltage variations.

in order to further facilitate the understanding of this level control system, specific values will be given to some of the components shown in FIG. 2 so that ranges within which the circuit operates will be made clearer. It should be pointed out that the component values are approximate and that the components have normal tolerances. Further, the particular set of values provided are for purposes of example only, there being a wide variety of sets of values which may be used under different conditions and for somewhat different results.

In the voltage divider, resistor 34 has a value of 10,000 ohms, resistor 35 has a value of 5,600 ohms, resistor 36 has a value of 390 ohms and the resistance values between probes l3 and 14 through fluid 23 to the shell of container 15 are assumed to be 1,000 ohms each when the fluid is above a certain level and both electrodes are immersed in the fluid. In this example, the fluid is assumed to be carbonated water. in this condition, with a standard electrical power of 117 v. applied by AC source 11, the peak output voltage across resistor 35 will be approximately 8.l5 v. When the fluid level decreases so that only electrode 13 is immersed, the peak output voltage of the voltage divider becomes approximately 16.5 v. When level of fluid 23 drops below a certain level so that both electrodes are dry, the peak output voltage becomes approximately 59.3 v. From this data it is evident that there need not be concern with minor differences in effective output impedance of the voltage divider since the threshold circuit may be constructed so that it is sensitive to only widely separated values of input voltage and is not affected by relatively minor variations from these predetermined levels.

In the demodulator, diode 37 is a standard small signal diode, capacitor 38 is a 50p.f. capacitor and resistor 39 is a 10,000 ohm resistor. In order to fully explain the function of resistor 39, it is necessary to consider, in part, power supply 31 and threshold detector 27. Power supply 31 is a conventional circuit and is constructed to hold the emitters of transistors 41 and 42 (typical production transistors) at some specific voltage level, taken here for purposes of this example to be 9.3 v. With the base-to-emitter voltage of transistor 41 being typically small, the omission of resistor 39 would allow the 9.3 v. DC level of the emitter to back-bias diode 37, thereby maintaining the voltage at the output of demodulator 26 equal to or less than 9.3 v. The result is, of course, that threshold detector 27 would not operate as desired, triggering on and off at entirely different points in the cycle. However, resistors 39 and 45 act together as a voltage divider with the result that the output of demodulator 26 is proportional to the AC output of voltage dividerl2. In response to theconditionsipreviously specified for a voltage divider :12, theoutput'of demodulator 26 will be approximately -2.8 v. with both electrodes immersed, 7 v. with the low electrode only immersed and -l2' v. with neither electrode immersed. It is thus apparent that resistor 39 assists in keeping the output voltage levels of demodulator 26 well separatedas well as preventing threshold detector 27 from back-biasingdiode 37.

The function of thresholdde'tector 27 is to translate the signals so that if both electrodesare dry, the motor should start and run until' both electrodes are immersed whenit will be turned off. Once, off,.the motor will not restartuntilboth electrodes again are dry. Aspreviously stated, the width of the hysteresis loop of threshold detector 27 depends upon the value of resistors 44 and 45 which are here taken to be 22,000 ohms each. The position at which threshold detector 27 triggers with respect to the value of input voltages is dependent upon the resistance value of resistor 46 which is heretake'n to be 56,000 ohms. With the component values givemdetector' 27 will trigger ON when the input voltage from demodulator 26 is approximately 9 v. and will trigger OFF when theinput voltage reaches 4 v.

It should be understood that the value of the output voltage of threshold detector 2 7 is not dependent upon :the. value of the input voltage from demodulator 26, butthat'the value of the input voltage determines whether or notdetector .27 is conducting.

in following a cycle of operation of the liquidlevel :control system described herein, thestartingrpoint is assumed to be with threshold detector 27 OFF and container full so that the output voltage of demodulator 26 is at the minimum level of 2'.8 v. As fluid level within container 15 decreases, the voltage level output of demodulator 26 decreases, that is, the absolute value of the voltage increases in a negative sense. As

the fluid level decreases, the'voltage decreases from 12.8 to

-7 v. when electrode 14 is no longer immersed in liquid23. Threshold detector 27 and power control means'32 are still off. Further reduction in liquid level uncovers low electrode l3,"causing the voltage of the threshold detector to dropbelow -9 v. At this point the voltage-on the gate 57 of triacr54 drops to .9.3 v. as the threshold circuit triggers, therebytriggering the triac and turning on .motor 33; Shortly'thereafter, low electrode 13 becomes covered with liquid and the input voltage to the threshold detectorl27 increases to 7 v. However, due to the feedback arrangement of the threshold detector circuit, this circuit remains ONand motor- 33 continues to run. As the liquid touches the high .level electrode, the input voltage to threshold detector 27 increases to 2.'8 v., thereby exceeding the trigger level of 4 v. for detector'27. Threshold detector 27 is turned off, the voltageon triac gate 57 increases to 0, and motor 33 is turned ofi.

By proper selection of resistance values for thevarious -resistors in the system, the output voltage levels for demodulator 26 may be kept well separated and the threshold detector made to operate within the desired limits. It should be remem- 'paratus;

variable; impedance voltage divider network connected' directly across said electrical power-applyingmeans,said

voltage divider network including:

said container;

the fluent material within said container;

a plurality of electrodes located within said container-at spaced levels; and

at least one resistor; the electrical signal output of said voltage divider network "being uniquely dependent upon the level 'of the fluent material in said container;

a demodulator circuit connected to said voltage divider network to convert the alternating current output signals thereof to directcurrent signals;

v a threshold detector for generating first and second indicating signals responsive to the direct current output signals of said demodulator circuit, said first and second indicating signals being nonuniquely dependent upon the level of the fluent material in said container;

means for producing control signals in response'to said first and second indicating signals; and means responsive to said control'signals for level of the fluent material in said container; said control signal-producing means and saidlevel-adjusting means being connected in series across said electrical power applying means.

2. The apparatus recited in claim 1, wherein:

said control signal-producing means includes a triode AC switch in series with said 'fluent material level-adjusting meansacross said electrical power-applying means, said triode AC switch being turned on inres'ponse tosaid first indicating signal to thereby cause said level-adjusting means to operate, and being turned off in response to said second indicating signal to thereby inhibit operation of said level-adjusting means.

3. The apparatus recited-in claim 1, wherein:

said electrodes are two in number, the first electrode being suspended within said container adjacent the bottom thereof, the second electrode being suspended within said container adjacent the top thereof.

4. The-apparatus recited in claim 1, wherein:

said threshold detector has a predetermined hysteresis characteristic whereby said first indicating signal is produced when the fluent material in said container is below a first predeten-nined level and said second indicating signal is produced when the fluent material in said container is above a second, predetermined level higher than said first predetermined level, one of said "first and second indicating signals being produced by said threshold detector when the fluent material is between said first and second predetermined levels, the determination of the indicating signal to be produced beingdependent upon the immediate past history of the 'level' of the fluent material in said container.

,5. The apparatus recited in claim 4, wherein:

said threshold detector produces said first indicating signal when the fluent material isbetween said first and second "adjusting the predetermined levels immediately after having been power supply connected to said electrical power-applying means, said power supply providing operating power voltages to said threshold detector.

7. The apparatus recitedin claim 6, wherein: said threshold detector comprises; a first transistor having its base coupled to theoutput of said demodulator circuit; e a second transistor having its base coupled to the collector of said first transistor the base of said first transistor being coupled to the collector of said second transistor,

the output of said power supply being coupled to the emitters of both said first and second transistors;

the collector of said second transistor being coupled to said control signal-producing means.

8. Apparatus for indicating changes in the level of a fluid within a container, said apparatus comprising:

means adapted for applying alternating electrical power to said apparatus;

a variable impedance voltage divider network connected directly to said electrical power-applying means, said voltage divider network including; said container; the fluent material within said container;

a plurality of electrodes located within said container at spaced levels; and

at least one resistor; the electrical signal output of said voltage divider network being uniquely dependent upon the level of the fluent material in said container;

a demodulator circuit connected to said voltage divider network to convert the alternating current output signals thereof to direct current signals;

a threshold detector for generating first and second indicating signals responsive to the direct current output signals of said demodulator, said first and second indicating signals being nonuniquely dependent upon the level of the fluent material in said container.

9. The apparatus recited in claim 8 and further comprising a power supply connected to said electrical power-applying means, said power supply providing operating power voltages to said threshold detector.

10. The apparatus recited in claim 8, wherein: said demodulator circuit and said voltage divider network having mutually proportional output signals, the value of said output signals being dependent upon the level of fluent material within said container, said output signals having a first predetermined value determined by a first predetermined level of the fluent material, a second predetermined value determined by a second predetermined level of the fluent material and a third predetermined value intermediate said first and second predetermined values and when the level of the fluent material is between said first and second predetermined levels; said threshold detector producing said first indicating signal in response to the signal of first predetermined value from said demodulator circuit and thereafter continuing to produce said first indicating signal when the level of the fluent material changes from said first predetermined level and until said level reaches said second predetermined level, said threshold detector producing said second indicating signal in response to said signal of second predetermined value from said demodulator circuit and thereafter continuing to produce said second indicating signal when the level of the fluent material changes from said second predetermined level and until said level reaches said first predetermined level. 

1. Apparatus for controlling the level of a fluent material in a container, said apparatus comprising: means for applying alternating electrical power to said apparatus; a variable impedance voltage divider network connected directly across said electrical power-applying means, said voltage divider network including: said container; the fluent material within said container; a plurality of electrodes located within said container at spaced levels; and at least one resistor; the electrical signal output of said voltage divider network being uniquely dependent upon the level of the fluent material in said container; a demodulator circuit connected to said voltage divider network to convert the alternating current output signals thereof to direct current signals; a threshold detector for generating first and second indicating signals responsive to the direct current output signals of said demodulator circuit, said first and second indicating signals being nonuniquely dependent upon the level of the fluent material in said container; means for producing control signals in response to said first and second indicating signals; and means responsive to said control signals for adjusting the level of the fluent material in said container; said control signal-producing means and said level-adjusting means being connected in series across said electrical power applying means.
 2. The apparatus recited in claim 1, wherein: said control signal-producing means includes a triode AC switch in series with said fluent material level-adjusting means across said electrical power-applying means, said triode AC switch being turned on in response to said first indicating signal to thereby cause said level-adjusting means to operate, and being turned off in response to said second indicating signal to thereby inhibit operation of said level-adjusting means.
 3. The apparatus recited in claim 1, wherein: said electrodes are two in number, the first electrode being suspended within said container adjacent the bottom thereof, the second electrode being suspended within said container adjacent the top thereof.
 4. The apparatus recited in claim 1, wherein: said threshold detector has a predetermined hysteresis characteristic whereby said first indicating signal is produced when the fluent material in said container is below a first predetermined level and said second indicating signal is produced when the fluent material in said container is above a second predetermined level higher than said first predetermined level, one of said first and second indicating signals being produced by said threshold detector when the fluent material is between said first and second predetermined levels, the determination of the indicating signal to be produced being dependent upon the immediate past history of the level of the fluent material in said container.
 5. The apparatus recited in claim 4, wherein: said threshold detector produces said first indicating signal when the fluent material is between said first and second predetermined levels immediately after having been below said first predetermined level, and produces said second indicating signal when the fluent material is between said first and second predetermined levels immediately after having been above said second predetermined level.
 6. The apparatus recited in claim 4 and further comprising a power supply connected to said electrical power-applying means, said power supply providing operating power voltages to said threshold detector.
 7. The apparatus recited in claim 6, wherein: said threshold detector comprises; a first transistor having its base coupled to the output of said demodulator circuit; a second transistor having its base coupled to the collector of said first transistor, the base of said first transistor being coupled to the collector of said second transistor, the output of said power supply being coupled to the emitters of both said first and second transistors; the collector of said second transistor being coupled to said control signal-producing means.
 8. Apparatus for indicating changes in the level of a fluid within a container, said apparatus comprising: means adapted for applying alternating electrical power to said apparatus; a variable impedance voltage divider network connected directly to said electrical power-applying means, said voltage divider network including; said container; the fluent material within said container; a plurality of electrodes located within said container at spaced levels; and at least one resistor; the electrical signal output of said voltage divider network being uniquely dependent upon the level of the fluent material in said container; a demodulator circuit connected to said voltage divider network to convert the alternating current output signals thereof to direct current signals; a threshold detector for generating first and second indicating signals responsive to the direct current output signals of said demodulator, said first and second indicating signals being nonuniquely dependent upon the level of the fluent material in said container.
 9. The apparatus recited in claim 8 and further comprising a power supply connected to said electrical power-applying means, said power supply providing operating power voltages to said threshold detector.
 10. The apparatus recited in claim 8, wherein: said demodulator circuit and said voltage divider network having mutually proportional output signals, the value of said output signals being dependent upon the level of fluent material within said container, said output signals having a first predetermined value determined by a first predetermined level of the fluent material, a second predetermined value determined by a second predetermined level of the fluent material and a third predetermined value intermediate said first and second predetermined values and when the level of the fluent material is between said first and second predetermined levels; said threshold detector producing said first indicatiNg signal in response to the signal of first predetermined value from said demodulator circuit and thereafter continuing to produce said first indicating signal when the level of the fluent material changes from said first predetermined level and until said level reaches said second predetermined level, said threshold detector producing said second indicating signal in response to said signal of second predetermined value from said demodulator circuit and thereafter continuing to produce said second indicating signal when the level of the fluent material changes from said second predetermined level and until said level reaches said first predetermined level. 